Active-passive electromagnetic shielding to reduce mri acoustic noise

ABSTRACT

The present invention provides an apparatus for reducing acoustic noise in a magnetic resonance imaging device including passive shielding located outside the actively shielded gradient winding elements in order to reduce the magnitude of fields that spread outside the gradient coil assembly in unwanted directions and interact with the magnet cryostat or other metallic magnet parts, inducing eddy currents that cause consequent acoustic noise. The passive shielding elements are conducting layers located on the outer radius of the cylindrical gradient coil assembly in a cylindrical magnet system, conducting layers located at the ends of the gradient coil assembly in a cylindrical magnet system, and conducting layers located inside the actively shielded gradient winding inner elements in a cylindrical magnet system. The passive shielding could also be located on separate structures that are vibrationally isolated from the magnet cryostat. The actively shielded gradient winding can also be extended to portions at the ends of the actively shielded gradient winding and further to portions inside the inner radius of the inner portion of the actively shielded gradient winding. The actively shielded gradient windings and passive shielding should be designed concurrently in order to substantially optimize the gradient linearity and reduce the eddy currents generated in metallic parts of the magnetic resonance imaging system.

This application claims the benefit of: U.S. Provisional Application No.60/497,267, filed Aug. 25, 2003; U.S. Provisional Application No.60/500,225, filed Sep. 5, 2003; and U.S. Provisional Application No.60/570,593, filed May 13, 2004; and U.S. application Ser. No.10/925,691, filed Aug. 25, 2004, which are hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a magnetic resonance imaging(MRI) scanner and more particularly to a low acoustic noise MRI scanner.

MRI scanners, which are used in various fields such as medicaldiagnostics, typically create images based on the operation of a magnet,a gradient coil assembly, and a radiofrequency coil(s). The magnetcreates a uniform main magnetic field that makes unpaired nuclear spins,such as hydrogen atomic nuclei, responsive to radiofrequency excitationvia the process of nuclear magnetic resonance (NMR). The gradient coilassembly imposes a series of pulsed, spatial-gradient magnetic fieldsupon the main magnetic field to give each point in the imaging volume aspatial identity corresponding to its unique set of magnetic fieldsduring an imaging pulse sequence. The radiofrequency coil applies anexcitation rf (radiofrequency) pulse that temporarily creates anoscillating transverse nuclear magnetization in the sample. This samplemagnetization is then detected by the excitation rf coil or other rfcoils. The resulting electrical signals are used by the computer tocreate magnetic resonance images. Typically, there is a radiofrequencycoil and a gradient coil assembly within the magnet.

Magnets for MRI scanners include superconductive-coil magnets,resistive-coil magnets, and permanent magnets. Known superconductivemagnet designs include cylindrical magnets and open magnets. Cylindricalmagnets typically have an axially-directed static magnetic field. In MRIsystems based on cylindrical magnets, the radiofrequency coil, thegradient coil assembly and the magnet are generallyannularly-cylindrically shaped and are generally coaxially aligned,wherein the gradient coil assembly circumferentially surrounds theradiofrequency coil and wherein the magnet circumferentially surroundsthe gradient coil assembly. Open magnets typically employ twospaced-apart magnetic assemblies (magnet poles) with the imaging subjectinserted into the space between the assemblies. This scanner geometryallows access by medical personnel for surgery or other medicalprocedures during MRI imaging. The open space also helps the patientovercome feelings of claustrophobia that may be experienced in acylindrical magnet design.

A gradient coil assembly comprises a set of windings in a supportstructure that produce the desired gradient fields. Such an assembly fora human-size whole-body MRI scanner typically weighs about 1000 kg. Thewindings consist of wires or conductors formed by cutting or etchingsheets of conducting material (e.g. copper) to form current paths togenerate desired field patterns. The wires or conducting coils or platesare themselves typically held in place by fiberglass overwindings plusepoxy resin.

Generally, the various components of the MRI scanner represent sourcesand pathways of acoustic noise that can be objectionable to the patientbeing imaged and to the operator of the scanner. For example, thegradient coil assembly generates loud acoustic noises, which manymedical patients find objectionable. The acoustic noises occur in theimaging region of the scanner as well as outside of the scanner. Knownpassive noise control techniques include locating the gradient coilassembly in a vacuum enclosure.

Large pulsed electrical currents, typically 200 A or more, with risetimes and durations typically in the submillisecond to millisecondrange, are applied to the windings. Because these windings are locatedin strong static magnetic fields (e.g., 1.5 T for a typical clinicalimager to much higher values for research systems), the currentsinteract with the static field and strong Lorentz forces are exerted ondifferent parts of the gradient coil assembly. These forces in turncompress, expand, bend or otherwise distort the gradient coil assembly.It will be readily understood by those skilled in the art that thefrequencies of the acoustic noise so generated will be in the audiorange. Typically there are strong components of noise from 50 Hz andbelow to several kHz at the upper end of the frequency range.

The magnet cryostat and other metallic parts of the magnet assembly arealso sources of vibration and acoustic noise. For a cylindrical magnet,the actively shielded gradient winding generally comprises three layers,one for each Cartesian direction (x, y and z), each layer typicallyconsisting of an inner, primary winding portion that creates a gradientfield in the imaging region plus an outer, concentric shielding windingportion that substantially reduces the fields outside the gradientassembly. Some of the gradient-produced fields leak through or aroundthe shielding. These leakage fields can induce eddy currents in themagnet metallic parts, for example, the cryostat inner bore. These eddycurrents in turn produce Lorentz forces on the cryostat inner boreleading to mechanical motion of the cryostat inner bore and consequentacoustic noise according to WA Edelstein et al., Magnetic ResonanceImaging 20, 155-163, 2002.

The shielded gradients described above are constructed with innergradient windings to produce gradient fields Gx, Gy and Gz of the mainBo field along axes x, y and z. They also have larger diameter shieldingwindings designed to substantially cancel external fields produce by theinner gradient windings so that the net fields outside the shieldingwindings are substantially zero. There are generally three separateinner windings, one for each of the directions x, y and z, and threeseparate outer, shielding windings, one for each of the directions x, yand z. The complete x-gradient winding consists of various parts,including inner and shielding windings, which are generally electricallyconnected to form a single electrical circuit. The configurations of y-and z-gradient windings are similar. Up to the present, the generalpractice has been to design the inner and shielding windings so thateach axis has a single layer of inner windings and a single layer ofshielding windings. Typically, the inner x-winding is electricallyconnected in series to the outer, shielding x-winding and the inner plusouter x-winding circuit is then powered by a single power supply. Thesame is true of the y- or z-winding.

FIG. 1A is a cross-sectional side view of a conventional activelyshielded gradient. 304 is the metallic inner bore of the magnetcryostat. 20 is the inner gradient windings and 30 is the outer,shielding gradient windings. 20 and 30 are supported in 102, a solid,nonconducting annular support structure.

However, the active gradient shielding is not perfect, as some pulsedmagnetic fields leak through and around the shielding windings, interactwith the magnet or cryostat structure and cause eddy currents in thosestructures. As shown in Schenck et al., U.S. Pat. No. 4,617,516, 1986,the z-gradient windings are generally made using a wire wound in onedirection from one end of the gradient to the center, and the wirereverses from the center outward. Substantial field can leak through thewidely spaced turns in the center. For all windings, field can leakaround the ends of the windings to interact with the cryostat or othermetallic parts of the magnet structure. It has been shown that eddycurrents in the metallic inner bore of the magnet cryostat producedLorentz forces on the inner bore leading to vibrations of the inner boreand consequent acoustic noise (WA Edelstein et al., Magnetic ResonanceImaging 20, 155-163, 2002.)

Some recent designs to improve gradient active shielding produceincreasingly complex gradient current patterns. The length of thesewindings must be limited to ensure efficient gradient operation.Compromises thereby incurred limit the effectiveness of the additionalshielding achieved. The recent quest for shorter length and rapidlychanging gradients has also tended to work against effective shielding.

The active gradient shields in cylindrical magnet MRI systems heretoforehas been confined to a cylindrical surface disposed concentricallyrelative to the magnet. This arrangement is not the best configurationto produce the most effective shielding.

Passive shielding alone will not provide adequate shielding and eddycurrent control because of its finite time constants. Multiple layers ofactive gradient shielding will have limited effectiveness because activegradient shielding must have discrete current paths. We are proposingthe use of passive gradient shielding in conjunction with activegradient shielding. One example of this approach is disclosed in Mulderet al., U.S. Pat. No. 6,326,788, 2001. FIG. 3 is a cross-sectional sideview of their design. It is similar to the conventional activelyshielded gradient shown in FIG. 1 with the addition of a passive shield210 consisting of a conducting layer on the outer radius of the solid,nonconducting annular cylindrical support structure 102. The idea ofpassive shielding acting in combination with active shielding can beextended further with substantially improved efficacy.

What is needed is a method of drastically reducing gradient-induced eddycurrents in the magnet cryostat and other magnet parts, and to mitigateconsequent vibrations, in order to substantially alleviate one of theprincipal sources of acoustic noise in MRI scanners.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for generatingmagnetic resonance images with reduced acoustic noise. In oneembodiment, such an apparatus includes a gradient coil assembly passiveshielding element including at least one conducting element outside theactively shielded gradient coil winding. The passive shielding elementis located outside the gradient winding elements and is positioned tosubstantially block leakage fields that can otherwise produce eddycurrents in the magnet cryostat and other magnet structures.

A first aspect of the invention is directed toward a method for reducingacoustic noise in a magnetic resonance imaging device, the methodcomprising the steps of providing a passive shielding element adjacentan actively shielded gradient coil winding of the device and ensuring anactive gradient shielding winding is insulated from the passiveshielding element, wherein a first portion of the passive shieldingelement is located adjacent an outer surface of the actively shieldedgradient coil winding and a second portion is located adjacent an end ofthe actively shielded gradient coil winding.

A second aspect of the invention is directed towards an apparatus forreducing acoustic noise in a magnetic resonance imaging device, theapparatus comprising an actively shielded gradient coil winding, anannular support structure substantially enclosing the actively shieldedgradient coil winding, and a passive shielding element adjacent theactively shielded gradient coil winding, wherein a first portion of thepassive shielding element is adjacent an outer surface of the activelyshielded gradient coil and a second portion is adjacent an end of theactively shielded gradient coil and positioned to form anon-perpendicular angle with the first portion.

A third aspect of the invention is directed toward an apparatus forreducing acoustic noise in a magnetic resonance imaging device, theapparatus comprising: an actively shielded gradient coil winding; anannular cylindrical support structure substantially enclosing theactively shielded gradient coil winding; and an electrically conductivepassive shielding element adjacent the actively shielded gradient coilwinding, wherein a first portion of the passive shielding element isadjacent an outer surface of the actively shielded gradient coil and asecond curvilinearly shaped portion is adjacent an end of the activelyshielded gradient coil.

The foregoing and other features of the invention will be apparent fromthe following more particular description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will becomeapparent from the following detailed description of the invention whenread with the accompanying drawings in which:

FIG. 1 is a cross-sectional side view of a prior art actively shieldedgradient assembly without passive shielding;

FIG. 2 is a cross-sectional side view of an entire magnetic resonanceimaging system showing magnet and a prior art gradient coil assembly;

FIG. 3 is a cross-sectional side view of a prior art actively shieldedgradient assembly with prior art passive shielding;

FIG. 4 is a cross-sectional side view of an actively shielded gradientassembly with passive shielding according to the invention;

FIG. 5 is a cross-sectional side view of an actively shielded gradientassembly with etended active shielding according to the invention;

FIG. 6 is a cross-sectional side view of an actively shielded gradientassembly with passive shielding according to the invention and extendedactive shielding according to the invention.

FIG. 7 is a cross-sectional side view of an actively shielded gradientassembly with passive shielding according the invention.

FIG. 8 is a cross-sectional side view of an actively shielded gradientassembly with passive shielding according the invention.

FIG. 9 is a cross-sectional side view of an actively shielded gradientassembly with passive shielding according the invention.

DETAILED DESCRIPTION OF INVENTION

Referring to FIG. 2 there is shown an illustrative MRI device 90 towhich embodiments of the present invention are applicable. MRI device 90is of a type useful in producing magnetic resonance (MR) images of apatient or subject. Throughout the figures, like numerals represent likeelements. FIGS. 1-6 show MRI device 90 based on a closed, cylindricalsuperconducting magnet assembly 200. It is to be appreciated by oneskilled in the art that the functions and descriptions of the presentinvention are equally applicable to an open magnet configuration.

FIG. 1 is a cross-sectional side view of a prior art actively shieldedgradient assembly. The actively shielded gradient coil winding consistsof an inner active gradient winding portion 20 and an outer gradientwinding portion 30. Generally these are electrically connected in seriesand powered by a single power amplifier. They could be powered by twoseparate power amplifiers. The active gradient coil winding 20 plus 30is embedded in an insulating support matrix 102, typically comprised offiberglass-epoxy or other filler materials.

Referring to FIG. 2, this type of cylindrical magnet assembly, withcenter axis 250, comprises an inner surface referred to as a magnetcryostat inner bore 304 and a cryostat shell 100 disposed radiallyaround the outer surface. The magnet assembly further comprises end capseals 212. When end cap seals 212 are secured against rubber gaskets 220positioned between end cap seals 212 and cryostat shell 100, and securedagainst other rubber gaskets 220 positioned between end cap seals 212and patient tube 104, an airtight region 106 containing the gradientcoil assembly 190 is created.

Typically, cryostat shell 100 encloses a superconductive magnet (notshown) that, as is well-known, includes several radially-aligned andlongitudinally spaced-apart superconductive coils, each capable ofcarrying a large electric current. The superconductive coils produce ahomogeneous, main static magnetic field, known as B₀, typically in therange from 0.5 T to 8 T, aligned along the center axis 250. Cryostatshell 100 is generally metallic, typically steel or stainless steel.

A patient or imaging subject (not shown) is positioned within acylindrical imaging volume 201 surrounded by patient bore tube 104. Boretube 304 is typically made of electrically conducting material such asstainless steel. Gradient coil assembly 102 is disposed around in aspaced apart coaxial relationship therewith and generates time-dependentgradient magnetic field pulses in a known manner. Radially disposedaround gradient coil assembly 102 is cryostat shell 100 including warmbore 304. Cryostat shell 100 contains the magnet that produces thestatic magnetic field necessary for producing MRI images, as describedabove.

Also shown in FIG. 2 is a schematic view of a vibration isolationsuspension arrangement consisting of bracket 112 solidly attached to themagnet cryostat 200, bracket 108 solidly attached to gradient assembly190. Elastomeric layer 110 supports the weight of gradient assembly 190and reduces the amplitude of vibrations transmitted from gradientassembly 190 to the magnet cryostat 200.

Leakage pulsed magnetic fields from the gradient assembly 190 createeddy currents in the cryostat inner bore 304. These eddy currentssubject the cryostat bore 304 to Lorentz forces which produce vibrationsand consequent MRI system acoustic noise as shown by WA Edelstein etal., Magnetic Resonance Imaging 20, 155-163, 2002.

FIG. 3 illustrates an approach by Mulder et al., U.S. Pat. No.6,326,788, 2001 to apply a passive shield in a position at a radiusbeyond the active gradient winding 20 plus 30. FIG. 3 is across-sectional side view of their design. It is similar to theconventional actively shielded gradient shown in FIG. 1 with theaddition of a passive shield 210 consisting of a conducting layerpositioned outside the active gradient winding 20 plus 30.

FIG. 4 shows a gradient assembly 190 with extended passiveelectromagnetic shield comprised of conductive portions 210, 220 and230. Portions 210, 220 and 230 should be a good electrical conductor,for example, copper. For the purpose of clarity, gaps are shown betweenportions 210 and 220 and gaps between conductive portions 220 and 230.However, in practice, 210 and 220 will generally be continuouslyelectrically connected, as will 220 and 230, so the entire passiveshield would consist of a single conducting layer consisting of portions210, 220 and 230. Fields that escape from the active shielding windings20 plus 30 would cause eddy currents in the passive shield 210 plus 220plus 230. These eddy currents would help contain fields inside thepassive shield and prevent those fields from interacting with thecryostat bore 304 or other metallic magnet system parts. It may beadvantageous in some situations, however, to have gaps as shown here orin other positions to better optimize the operation of the gradientassembly 190.

FIGS. 7-8 show gradient assembly 190 with an extended passiveelectromagnetic shield comprising conductive portions 210, 222 and 232.While a wide range of materials may be used, it is preferable thatportions 210, 222 and 232 are made from a highly conductive material,such as copper. For the purpose of clarity, gaps are shown betweenportions 210, 222 and 232. However, in practice, portions 210, 222 and232 are continuously electrically connected, so that the entire passiveshield consists of a single conducting layer. During operation, fieldsthat escape from the active shielding windings 20, 30 cause eddycurrents in passive shields 210, 222 and 232. The eddy currents helpcontain fields inside the passive shield and prevent those fields frominteracting with the cryostat bore 304 and other metallic magnet systemparts. It may be advantageous in some situations, however, to have a gapbetween nonconducting annular cylindrical support structure 102 and thepassive shield, in order to better optimize the operation of thegradient assembly 190. In the present invention, a first portion of thepassive shielding element is located adjacent an outer surface of theactively shielded gradient winding and a second portion is locatedadjacent an end of the actively shielded gradient winding, wherein thesecond portion forms a non-perpendicular angle with the first portion ofthe passive shielding element.

As shown in FIG. 7, in one embodiment of the present invention, portion232 is located adjacent the end of the actively shielded gradientwindings 20, 30 and positioned to form angle 242 (θ) with portion 210.While angle 242 may be any angle, in a preferred embodiment, angle 242is one of an acute and an obtuse angle. Furthermore, it should beunderstood that angle 242 may be perpendicular relative to portion 210.However, in the present embodiment, second portion 222 forms anon-perpendicular angle with first portion 210 of the passive shieldingelement. Third portion 232 of the passive shielding element is adjacentan inner surface of the actively shielded gradient coil windings 20, 30.Third portion 232 contains a first section and a second section, whereinthe second section extends beyond the end of the actively shieldedgradient coil and is angled to meet the second portion. As shown, thefirst section is adjacent to annular cylindrical support structure 102and oriented substantially parallel to portion 210. The second sectionextends beyond the end of support structure 102 and is angled to meetsecond section 222. It can be appreciated by those skilled in the art,that an adjustment to angle 242 would influence the geometry of thesecond section. Also, although portion 222 is shown as beingsubstantially straight, a person skilled in the art would recognize thata number of other geometries are possible.

As shown in FIG. 8, angle 242 may be obtuse relative to an axis formedalong portion 210. In this embodiment, second portion 222 andcylindrical support structure 102 form an obtuse angle relative to thefirst portion 210. As shown by this embodiment, support structure 102may take on a variety of geometries/dimensions. Third portion 232contains a first section and a second section, wherein the secondsection extends beyond the end of the actively shielded gradient coiland is angled to meet the second portion. As shown, the first section isadjacent to annular cylindrical support structure 102 and orientedsubstantially parallel to portion 210. The second section extends beyondthe end of support structure 102 and is angled to meet second section222. Furthermore, although not shown, second portion may be positionedat an obtuse angle in the case that cylindrical support structure 102 ishollow. Although portion 222 is shown as being substantially straight, aperson skilled in the art would recognize that a number of othergeometries are possible.

FIG. 9 shows another embodiment of gradient assembly 190 with extendedpassive electromagnetic shield comprised of conductive portions 210, 224and 234. In this embodiment, a first portion of the passive shieldingelement is adjacent an outer surface of the actively shielded gradientcoil and a second curvilinearly shaped portion is adjacent an end of theactively shielded gradient coil. As shown, portions 224 and 234 arecurved and may be positioned at angle 244 relative to portion 210. Itshould be understood that angle 244 may be any angle. However, in thepresent embodiment, second portion 222 forms a non-perpendicular anglewith first portion 210 of the passive shielding element. As also shown,third portion 234 of the passive shielding element is adjacent an innersurface of the actively shielded gradient coil winding, wherein thirdportion 234 contains a first section and a second section, and whereinthe second section extends beyond the end of the actively shieldedgradient coil and is curvilinearly shaped to meet second portion 224.Although portions 224, 234 are shown as being curvilinearly shaped andpositioned at angle 244, a person skilled in the art would recognizethat a number of other geometries and angles are possible. Finally,although shown in FIGS. 4-9 as having circular symmetry about thecentral symmetry axis 250 of the gradient assembly and reflectivesymmetry relative to the midplane of the gradient assembly, otherembodiments having an asymmetric passive shielding portion are possible.

The additional conductive portions improve shielding efficacy. This isdemonstrated by WA Edelstein et al., Proc. Intl. Soc. Mag. Reson. Med.11, 747 (2004), where a comparison is made between the efficacy of apassive shield such as portion 210 only and a passive shield whichconsists of portions 210 and 220. For a 1 mm passive shield and a seriesof repeated trapezoidal pulses, and relative to a configuration with nopassive shield, energy deposited in the cryostat bore 304 is reduced14.7 dB with a passive shield consisting of portion 210 only and by 19.9dB with a passive shield consisting of portions 210 and 220.

If a passive shield 210, 220 and 230 is positioned outside the gradientassembly, then any forces on this final shield may cause motion of theshield. However, if the passive shield is firmly mounted on the gradientassembly, its motion will be limited as compared to the motion of a thincylinder such as the cryostat bore. Vibrational isolation of thegradient assembly 90 will prevent vibrations from the gradient assemblyor passive shield from being conveyed to the rest of the MRI system andthen causing acoustic noise. Finally, if a vacuum is created around thegradient assembly in airtight region 106, then motion of the gradientassembly or passive shield cannot cause acoustic noise to be conveyedthrough air.

The presence of passive shielding elements 210, 220 and 230 will alterthe field distributions when active gradient winding 20 plus 30 ispulsed. Therefore it is important to design the active gradient winding20 plus 30, and the passive shielding elements 210, 220 and 230concurrently, in order to optimize the disposition of pulsed gradientfields in imaging region 201 and minimize the eddy currents created incryostat inner bore 304 and other metallic magnet system parts.

The active shielding can also be extended to enhance its function asshown in FIGS. 5 and 6. FIG. 5 shows actively shielded gradient coilwinding portion 30 extended to end portions 32 and, if advantageous,extended further to cylindrical portions 34. For the purpose of clarity,gaps are shown between portions 30 and 32 and gaps between conductiveportions 32 and 34. However, in practice, 30 and 32 will generally becontinuously electrically connected, as will 32 and 34, so the activelyshielded gradient coil winding portion for each axis comprises a singleconducting layer consisting of: portions 30 and 32; or portions 30 and32 and 34. It may be advantageous in some situations, however, to havegaps as shown here or in other positions, so that different activegradient portions have separate power supplies, to better optimize theoperation of the gradient assembly 190.

FIG. 6 shows a combination of extended actively shielded gradient coilwinding and extended passive shield.

Acoustic noise will also be reduced if the passive shield is mounted ona separate structure, also vibrationally isolated and contained in avacuum, outside the gradient assembly but separate from thecryostat/magnet structure. Reduction of eddy currents in the cryostatinner bore effected by the passive shield will reduce bore vibrationsand consequent acoustic noise.

There may be additional undesirable eddy currents generated by theactive outer shield or passive shield that lead to undesirable fields inthe imaging region. For example, misalignment of the inner and outergradient windings tends to produce extra, pulsed, uniform B0 magneticfields (as opposed to gradient fields) synchronized with the pulsedgradient fields. This already happens in typical MRI systems because ofinteraction of misaligned gradient windings with the cryostat innerbore. It may therefore be desirable to include in this gradient packageadditional windings (uniform field, second order, third order, fourthorder, etc.) that can be pulsed in order to cancel unwanted fields.These extra windings could be mounted on the inner actively shieldedgradient winding portion 20, the outer actively shielded gradientwinding portion 30, or anywhere at a radius less than that of thepassive shielding 210 but at a radius greater than that of the gradientassembly inner bore. If necessary, a primary BO (uniform field) winding,for example, could have an associated shielding BO winding on the inneror outer windings. Higher-order windings and corresponding shieldingwindings could also be installed.

The initial magnitude of the eddy currents and consequent fields in theimaging region 201 will not change dramatically when supplementarypassive shields 210, 220 and 230 are introduced. This is because thepassive supplementary shields are at only a slightly smaller radius thanthe cryostat inner bore 304. However, using a copper passive shield, forexample, will substantially increase the time constants of the eddycurrents as compared to the time constants of eddy currents on thestainless steel bore. This is advantageous because longer timeconstants, and consequent slower variations of interfering fields, areeasier to compensate than rapidly changing interfering fields.

An additional benefit of reducing leakage of pulsed magnetic fields isthat eddy currents within the magnet structure will be reduced, therebylessening power dissipation and undesirable heating of the magnetelements. Typically, the magnet elements are kept at very lowtemperatures by means of refrigeration or cryogenic fluids. Additionalheating undesirably reduces efficiency of the magnet and increasesoperating costs.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the embodiments of the invention as set forth aboveare intended to be illustrative, not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the following claims.

1. A method for reducing acoustic noise in a magnetic resonance imagingdevice, the method comprising the steps of: providing an electricallyconductive passive shielding element adjacent an actively shieldedgradient coil winding of the device contained within an annular supportstructure; and ensuring the actively shielded gradient coil winding isinsulated from the passive shielding element, wherein a first portion ofthe passive shielding element is located adjacent an outer surface ofthe actively shielded gradient winding and a second portion is locatedadjacent an end of the actively shielded gradient winding.
 2. The methodof claim 1, wherein the second portion forms a non-perpendicular anglewith the first portion of the passive shielding element.
 3. The methodof claim 1, wherein the second portion is curvilinearly shaped.
 4. Themethod of claim 1, wherein the providing step includes providing a thirdportion of the passive shielding element located adjacent an innersurface of the actively shielded gradient winding.
 5. The method ofclaim 4, wherein the third portion has a first section and a secondsection, and wherein the second section extends beyond the end of theactively shielded gradient winding and is shaped to meet the secondportion.
 6. The method of claim 4, wherein the first and second portionsare electrically connected, and wherein the third portion iselectrically connected to at least one of the first and second portions.7. The method of claim 1, wherein the providing step results in a designfor the actively shielded gradient winding and the passive shieldingelement that substantially optimizes at least one of the gradientlinearity and the minimized generation of eddy currents in metallicparts of the magnetic resonance imaging device.
 8. An apparatus forreducing acoustic noise in a magnetic resonance imaging device, theapparatus comprising: an actively shielded gradient coil winding; anannular cylindrical support structure substantially enclosing theactively shielded gradient coil winding; and an electrically conductivepassive shielding element adjacent the actively shielded gradient coilwinding, wherein a first portion of the passive shielding element islocated adjacent an outer surface of the actively shielded gradient coiland a second portion is located adjacent an end of the actively shieldedgradient coil and positioned to form a non-perpendicular angle with thefirst portion of the passive shielding element.
 9. The apparatus ofclaim 8, wherein a third portion of the passive shielding element isadjacent an inner surface of the actively shielded gradient coilwinding.
 10. The apparatus of claim 8, wherein the third portioncontains a first section and a second section, wherein the secondsection extends beyond the end of the actively shielded gradient coiland is angled to meet the second portion.
 11. The apparatus of claim 10,wherein the first and second portions are electrically connected, andwherein the third portion is electrically connected to at least one ofthe first and second portions.
 12. The apparatus of claim 8, wherein aportion of the passive shielding element is electrically insulated froma portion of the actively shielded gradient coil winding.
 13. Theapparatus of claim 8, wherein the actively shielded gradient winding andthe passive shielding element are designed to substantially optimize atleast one of the gradient linearity and the minimized generation of eddycurrents in metallic parts of the magnetic resonance imaging device. 14.An apparatus for reducing acoustic noise in a magnetic resonance imagingdevice, the apparatus comprising: an actively shielded gradient coilwinding; an annular cylindrical support structure substantiallyenclosing the actively shielded gradient coil winding; and anelectrically conductive passive shielding element adjacent the activelyshielded gradient coil winding, wherein a first portion of the passiveshielding element is located adjacent an outer surface of the activelyshielded gradient coil and a second curvilinearly shaped portion islocated adjacent an end of the actively shielded gradient coil.
 15. Theapparatus of claim 14, wherein a third portion of the passive shieldingelement is adjacent an inner surface of the actively shielded gradientcoil winding.
 16. The apparatus of claim 15, wherein the third portioncontains a first section and a second section, wherein the secondsection extends beyond the end of the actively shielded gradient coiland is curvilinearly shaped to meet the second portion.
 17. Theapparatus of claim 15, wherein the first and second portions areelectrically connected, and wherein the third portion is electricallyconnected to at least one of the first and second portions.
 18. Theapparatus of claim 14, wherein a portion of the passive shieldingelement is electrically insulated from a portion of the activelyshielded gradient coil winding.
 19. The apparatus of claim 14, whereinthe actively shielded gradient winding and the passive shielding elementare designed to substantially optimize at least one of the gradientlinearity and the minimized generation of eddy currents in metallicparts of the magnetic resonance imaging device.